The ion plasma electron gun of the present invention is of the same general type as the gun disclosed in U.S. Pat. No. 3,970,892 and U.S. Pat. application Ser. No. 596,093. As stated in the prior art, a recent advance in the art of generating high energy electron beams for use, for example, in e-beam excited gas lasers, is the plasma cathode electron gun. In such an electron gun, a plasma is produced in a hollow cathode discharge between the hollow cathode surface and an anode grid operated at a relatively low voltage with respect to the cathode. Electrons are extracted from the discharge plasma through the anode grid and a control grid, and these electrons are accelerated to high energies in a plasma-free region between the grids and an accelerating anode which, typically, is a thin foil window maintained at a relatively high voltage with respect to the cathode. Among the advantages of the plasma cathode electron gun are its structural simplicity and ruggedness, high controllability and efficiency, low cost, and suitability for producing large area electron beams. A second patent, namely, U.S. Pat. No. 4,025,818, discloses an ion plasma electron gun which is essentially similar to the gun described above with the addition of a wire anode for the plasma discharge and a number of baffles which allow the plasma chambers to be connected sequentially. In prior electron beam generators, the beam current is generally proportional to the current provided to the cathode by the high voltage power supply. Thus, controlling and maintaining a uniform and constant dose rate is simply a question of measuring and controlling the current supplied to the cathode.
In the case of wire ion plasma devices, however, the high voltage power supply current is the sum of incident helium ions generated in the plasma chamber and the electrons emitted from the cathode surface. The ratio of emitted electrons to incident ions, the secondary emission coefficient, is dependent upon the surface conditions on the emitter surface. These conditions have been found to be quite changeable, so that the mere monitoring of the high voltage power supply current is inadequate for controlling and maintaining constant the dose rate or secondary electron output of the wire ion plasma device.
It is thus an object of the present invention to provide a means for accurately controlling and maintaining constant the dose rate or secondary electron beam output of a wire ion plasma electron gun.